1. Field of the Invention
The present invention relates to a thickness reduction inspection method/apparatus for detecting a thickness reduction phenomenon of such as a pipe by an ultrasonic flaw detection method which is one of nondestructive inspection techniques.
2. Description of the Related Art
In recent years, since we have experienced rupture of a large diameter pipe due to thickness reduction in a nuclear power plant, a thickness reduction inspection by an ultrasonic thickness measurement method is simultaneously performed to corresponding portions of pipes in thermal power plants and nuclear power plants.
Inspection target portions and measurement methods are strictly managed based on technical codes established by The Japan Society of Mechanical Engineers (JASME), “technical codes for pipe wall thickness reduction management in Power Generation Facility Codes of JASME. Regarding a pipe portion which may cause thickness reduction, a facility manager measures, according to standards, a wall thickness in an axial direction and a circumferential direction at intervals of several tens mm based on multiple reflection echoes on a pipe inner surface.
Pipe thickness reduction phenomena to be measured are flow accelerated corrosion (FAC) and droplet impact erosion. FAC is a thickness reduction phenomenon in which corrosion is accelerated by facilitating mass transfer by flow near a pipe wall surface under flow conditions of a water single phase flow or a two phase flow forming a liquid film on a pipe wall.
When FAC occurs, pipe thickness reduction is gradually expanded over a wide range, and in the case where appropriate management is not performed, the FAC causes pipe rupture. Therefore, the appropriate management is critically important in thickness reduction management.
On the other hand, droplet impact erosion is a phenomenon in which the thickness of a pipe material is reduced by an impact force generated when droplets collide at a high speed on a pipe wall surface in a high-speed two-phase flow line.
In a nuclear plant, many pipes to be inspected are disposed at a high place where a scaffolding is needed, and also a heat insulating material is wound up around the pipes. Therefore, in the case where the thickness reduction inspection is performed, the inspection is performed after setting the scaffolding and removing the heat insulating material. Further, after measuring, it is necessary to wind up the heat insulating material again and remove the scaffolding.
Such work needs a large amount of labor and time and has a significant impact on inspection operation costs.
Therefore, a new inspection method is desired to reduce inspection items. The reduction of pipe inspection items is an issue common in industrial fields using a pipe such as the oil field and the gas field.
In recent years, various apparatuses and methods are proposed to efficiently inspect a pipe wall thickness and a pipe damage by using ultrasonic waves.
For example, JP 2015-055559 A discloses a method in which at least three ultrasonic transceivers are discretely provided to a pipe, and a position and a size of a damage and a scale adhered in the pipe due to corrosion and erosion generated in the pipe are measured based on ultrasonic wave signals transmitted and received among these transceivers.
According to JP 2015-055559 A, pulse ultrasonic waves output from one ultrasonic transceiver reach to a damage by propagating on a pipe wall of the pipe, and a part of the ultrasonic waves is reflected at the damage. Further, the reflected waves return to the ultrasonic transceiver, and also the reflected waves are detected in ultrasonic transceivers disposed at the other places. Then, based on a propagation time of the reflected waves, a position of a damage can be calculated by a method such as triangulation.
In addition, as a method to detect pitting corrosion of a pipeline laid underground or at the bottom of the sea, JP 61-111462 A discloses a pitting corrosion detection method in which a plurality of sensors and pulsars is connected to a pair of measuring circuits, the pulsars are sequentially driven at certain time intervals, in synchronization with this, the sensors are sequentially connected to the measuring circuits in a time division manner, and consequently several points can be detected by a pair of measuring apparatuses.
The sensors and the pulsars are mounted in an apparatus moving in a pipe called an inspection pig. According to this method, several sensors and pulsars can be mounted to the inspection pig by reducing a number of measuring circuits, and pitting corrosion of a large-diameter pipeline can be detected at a small pitch.